Add "k3s certificate check" clause for better test coverage #9012

Job	Run time
build / Build	6m 38s
Integration Tests (certrotation)	1m 35s
Integration Tests (cacertrotation)	1m 31s
Integration Tests (etcdrestore)	1m 45s
Integration Tests (localstorage)	1m 29s
Integration Tests (startup)	5m 56s
Integration Tests (custometcdargs)	1m 0s
Integration Tests (etcdsnapshot)	8m 2s
Integration Tests (kubeflags)	6m 42s
Integration Tests (longhorn)	2m 53s
Integration Tests (secretsencryption)	2m 23s
Integration Tests (flannelnone)	1m 21s
	41m 15s

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